1. Field of the Invention
The present invention relates to a non-woven fabric made of tows and to a method of producing the same. The non-woven fabric has a high weight, is high in strength in all directions and can be used in manufacturing three-dimensional moulded products or as geotextile, for instance.
2. Description of the Related Art
Geotextiles have been used in the form of woven fabrics in those fields where marked strength is required, for example in reinforcement of an embankment or soft ground.
However, heavy weight woven fabrics can be produced only at a slow rate and require the use of yarns, which increases the cost of production thereof. From the viewpoint of physical properties, they have high strength in the machine and cross-machine directions but are weak in oblique directions at 45xc2x0. Therefore, they are not always suited for use as geotextiles, which are required to be strong in every direction. Thus, it becomes necessary to use unreasonably heavy woven fabrics. In this respect also, increases in cost result. To achieve improvement in this respect, tetraaxially woven fabrics have been developed and the use thereof as geotextiles has been studied (xe2x80x9cSangyo-yo Senishizai Handbook (Handbook of Textile Materials for Industrial Use)xe2x80x9d, published 1994 by Maruzen, page 376). However, tetraaxially woven fabrics are more difficult to produce than woven fabrics produced on ordinary weaving machines, which increases cost. They are thus unsuited for use as geotextiles for which cost is an important factor. Furthermore, woven fabrics, though high in strength, show a low extension ratio, hence their break energy (strengthxc3x97extension ratio) is low.
When geotextiles are used in other important fields, such as soaking up water, filtration or drainage, for instance, they are required to have water permeability and, accordingly, spunbonded non-woven fabrics and short fiber non-woven fabrics have so far been used. However, spunbonded non-woven fabrics have an average fiber diameter as large as 25 to 30 micrometers, hence their water permeability, which depends on the capillary action of fibers, is not always high. On the other hand, spunbonded non-woven fabrics produced from fibers of a smaller diameter are poor in productivity and cannot avoid increases in cost. Furthermore, the filaments of spunbonded non-woven fabrics are weak in strength, namely their strength is only about 20 mN/tex. Further, the filament alignments in such fabrics are random, hence the capillary action utilization efficiency is low. The short fibers used in non-woven fabrics have a fiber strength as high as 100 to 200 mN/tex but the strength of the non-woven fabrics, which depends on entanglement of the fibers, is very low because the fibers are short. Furthermore, the fiber alignment in the non-woven fabrics is random, and this is not efficient from the viewpoint of water permeability.
In the above discussion, geotextiles have been taken as an example. However, the foregoing comments also apply to the prior art problems in other fields where high tenacity and high break energy are required, for example, roofing base cloths, carpets and flexible containers.
The present inventors have sought to alleviate the weak strength, in oblique directions of 45xc2x0, of conventional woven fabrics and cross laminated non-woven fabrics and to develop multiaxially laminated non-woven fabrics, for example triaxially and obliquely laminated or tetraaxially laminated ones (U.S. Pat. No. 5,308,424 and U.S. Pat. No. 5,338,593). Since, however, the fabrics of these laminates are made of yarns, only coarse textures can be realized and the productivity in manufacturing products having a high unit weight is poor. Further, according to these prior patents, the mutual bonding of yarns depends on an adhesive. This means an increased cost of production. Further, the adhesive bonding is generally weak in strength and the adhesive has no heat resistance and, therefore, the utility of the products is limited in many instances.
For producing non-woven fabrics having a great basis weight of 1 kg/m2, for instance, from conventional non-woven fabrics such as spunbonded non-woven fabrics or short fiber-based non-woven fabrics, it is necessary to markedly reduce the production speed. Thus, the advent of a technology for efficiently producing non-woven fabrics having a high unit weight, at low cost has been desired.
In the field of plastics, thermal forming of plastic sheet is employed to produce moulded three-dimensional products. However, woven fabrics have no thermal formability and show a low extension ratio, so that they cannot be used in three-dimensional products.
Some non-woven fabrics have thermal formability (Japanese Laid-open Patent Publication Nos. Sho 60-199957, Sho 60199961, and Hei 8-291457). However, they are formed of low molecular oriented filaments and therefore the finished products made thereof are low in strength and dimensional stability. These are basically spunbonded non-woven fabrics, and hence the productivity in obtaining products with a high unit weight is low.
Therefore, a need exists for materials capable of being made into three-dimensional moulded products, such as car panels, ceilings, office reception room sofas and the like, having high weight and resembling cloths in feel and touch.
Furthermore, the disposal of waste plastics such as PET bottles is not only a problem in the plastics industry but also a social problem, involving the relevant administrative agencies and distribution sectors. The present invention has an important social significance in that such waste plastics are used as raw material resins to thereby open up a road to the mass use of geotextiles and the like while efficiently utilizing waste plastics.
Further, in ordinary tow production, spun and unstretched tows are stored in boxes or cans. A multiplicity of unstretched tows are drawn out from a number of such cans and subjected to stretching and crimping to give product tows or to directly give short fibers upon further processing. In such a process, a large number of cans are required for storing spun and unstretched tows and for supplying them to the step of stretching. Thus, a tow manufacturing plant gives an impression of being full of such cans, and efficiency from the viewpoint of plant space is low. Therefore, a rational method for producing tows, which might appropriately be used in the practice of the invention, is demanded.
Opened tow-based, cross-laminated, non-woven fabrics, produced by opening tows to thin webs, cross laying the webs and bonding them are disclosed in Japanese Laid-open Patent Publication No. Sho 52-124976. However, the efficiency of opening tows is not very good. While the opening of tows may be effective in producing non-woven fabrics low in basis weight, it is not efficient in producing non-woven fabrics with a high basis weight or non-woven fabrics strong in many directions.
It is an object of the present invention to provide a non-woven fabric having desired characteristics, which cannot have been attained with the conventional woven fabrics or non-woven fabrics, as well as a method of producing the same.
To accomplish the above object, the multiaxially laminated non-woven fabric of the invention has layers each composed of a multiplicity of parallel tows having a total tex of not less than 1,000 but not more than 30,000, and each of said tows is formed of crimped filaments having an average fineness of not more than 3 tex.
The above multiaxially laminated non-woven fabric is a tow-based, laminated, non-woven fabric resulting from bonding by at least one of the following techniques or methods (1) to (5): (1) Needle punching, (2) stitch bonding, (3) ultrasonic bonding, (4) water jet, (5) through air.
The above multiaxially laminated, non-woven fabric shows a tensile strength at 50% elongation not less than 10 mN/tex in each of warp direction, weft direction and oblique direction at45xc2x0.
The tows constituting the above multiaxially laminated, non-woven fabric comprise tows composed of conjugate fibers or bicomponent fibers made of at least two polymers differing in softening point, and are bonded together by thermal embossing or thermal press treatment to give a tow-based, laminated, non-woven fabric.
Another object of the present invention is to provide three-dimensional moulded products made of the above tow-based, laminated, non-woven fabric.
A further object of this present invention is to provide a method of producing tow-based multiaxial non-woven fabrics in which multiaxial lamination of tows is followed by interfilament bonding or web heat treatment in a state in which oblique members multiaxially overlaid and obliquely crossing one another are held, at opposing side edges thereof, by pins.
A still further object of this present invention is to provide a method of producing three-dimensional moulded products made of a tow-based laminated non-woven fabric which comprises shaping or moulding the above multiaxially laminated, non-woven fabric comprising conjugate fibers or bicomponent fibers, made of at least two polymers differing in softening point, into three-dimensional moulded products and then subjecting the three-dimensional moulded products to heat treatment at a temperature not lower than the softening point of the polymer which has the lower softening point.